1. Field of the Invention
This invention relates to a thin-film formation system and a thin-film formation process. More particularly, this invention relates to a thin-film formation system having a chamber consisting basically of a sputtering space and a film-forming space and a grid plate disposed between the sputtering space and the film-forming space, and a thin-film formation process making use of this system.
2. Related Background Art
Sputtering, which is one of thin-film formation processes, is a thin-film formation process in which positive ion particles generated by glow discharge are made incident on a film material to which a negative potential is kept applied, called a target, and a film material which have been released to the vacuum, called sputtered particles, are deposited on a substrate by the phenomenon of sputtering.
The sputtering, compared with vapor deposition, can form films having superior characteristics because the sputtered particles have a higher kinetic energy. It also has a characteristic feature that the system can be of simple construction and has a superior process reproducibility even in comparison with other processes in which a high energy is used to make material particles incident on substrates, such as ion plating. For these reasons, it is one of film formation processes most frequently used at present in the industrial production of thin films, e.g., in film coating of glass and film formation processes for semiconductor devices.
Conventional sputtering, however, utilizes glow discharge generated between the cathode target and the anode chamber, and hence the inside of a vacuum chamber has had to be filled with a rare gas of hundreds of mPa in pressure, called sputtering gas.
FIG. 1 illustrates the construction of a conventional thin-film formation system which carries out common sputtering. A vacuum chamber 2 shown here is provided therein with a target 3 and a substrate 8 on which a film is to be formed which are so disposed as to face each other substantially in parallel. The target 3 is fastened to a cathode member for magnetron sputtering. This cathode member has a backing plate 4 to which the target is fastened and a negative voltage is applied from a DC power source, a plurality of magnets which generate a magnetic field in the plane substantially parallel to the target 3, a shield plate 5 which protects members other than the target 3 from the phenomenon of sputtering, and a cooling-water circulator 10 for cooling the target 3.
A vacuum pump 1 is attached to the vacuum chamber 2. To the vacuum chamber 2, argon gas is introduced through a flow controller 9. Reference numeral 11 denotes argon molecules and 12, sputtered particles.
As can be seen from this drawing, the substrate 8 and target 3 which are disposed in the vacuum chamber 2 are always exposed to sputtering gas in the course of film formation.
Meanwhile, J. Vac. Sci, Technol., Vol. 11, No. 4, July/August 1974, pp. 666-670 refers briefly to the relationship between film characteristics and sputtering gas in the above conventional thin-film formation system. This paper explains that, with an increase in the pressure of sputtering gas, films come to have many pores (voids) and have columnar structure. The films having many voids and having columnar structure are physically weak and also chemically unstable, and hence are unfit for industrial use in many cases.
Electrochimica Acta 44 (1999), pp. 3945-3952, also reports the relationship between the pressure of sputtering gas, the incident angle xcex8 of sputtered particles on substrate and the kinetic energy of sputtered particles. This paper explains that, when the sputtering gas is at a high pressure, the sputtered particles have a low kinetic energy and falls on the substrate at a large incident angle xcex8.
In general, film material particles having a low kinetic energy move on the substrate surface at a short distance after their collision against it, and have a low possibility of becoming stable at optimum positions of the film surface. Also, with an increase in particles falling at a large incident angle xcex8 and incident obliquely on the substrate, films come to have many voids or pores because of the shadowing effect ascribable to the unevenness of substrate surface.
As stated above, when the sputtering gas is at a high pressure, the sputtered particles released from the target are scattered by the sputtering gas before they reach the substrate, and fall on the substrate at a large incident angle xcex8, so that the film material particles lose their kinetic energy correspondingly to cause deterioration of film quality inevitably.
To solve such problems, it has been considered necessary to develop a method of lowering the pressure of sputtering gas in the course of film formation or removing the scattering effect of sputtered particles that is caused by sputtering gas.
Sputtering which can be carried out at a low pressure includes, e.g., a low-voltage sputtering process disclosed in Shinku, Vol. 35, No. 2 (1992), pp. 70-75, and a self-sputtering process disclosed in J. Vac., Sci. Technol. A11(6), November/December (1993), pp. 2980-2984.
The low-voltage sputtering is a process in which the potential applied to the target and the magnetic-field intensity at the target surface are made higher to make the density of plasma higher so that the glow discharge can be continued even at a low pressure. The self-sputtering is a process in which the plasma is formed using film material particles in place of sputtering gas, and is a process by which the plasma can be formed without feeding any sputtering gas into the vacuum chamber.
These two processes, however, involve problems in practical use such that the both can make glow discharge stable with difficulty, have a low film formation rate, and, compared with conventional sputtering, afford no free selection of film materials.
Japanese Patent Application Laid-Open No. 6-192829 also discloses a process in which a sputtering chamber and a film-forming chamber are separately set up and the sputtered particles are drawn into the film-forming chamber through holes which connect the sputtering chamber and the film-forming chamber. This process enables the pressure in the film-forming chamber to be set lower than the sputtering chamber, and hence can keep the film material particles from scattering in the film-forming chamber. However, the inside of the sputtering chamber is filled with sputtering gas kept at substantially the same pressure as in the case of conventional sputtering, and film material particles scattered in the sputtering chamber reach the substrate with no rectification. Thus, this is not a process by which the sputtered particles can be kept from scattering so that their incident angles on the substrate can be made perfectly uniform.
Collimation is available as a sputtering process taking note only of the rectification of sputtered particles, which is carried out without lowering the pressure of sputtering gas. The collimation is a process in which a collimator is provided between a substrate and a target to control the incident angles of sputtered particles on the substrate. This process, however, has a possibility that the particles having passed the collimator are unwantedly scattered by the sputtering gas before they reach the substrate.
As discussed above, any conventional sputtering can not completely remove the influence of the scattering of sputtered particles that is caused by sputtering gas, and it has been difficult to produce dense and high-quality thin films having a good bottom coverage.
An object of the present invention is to solve the above problems the prior art has had, and provide a process and a system which are able to form dense and high-quality thin films having a good bottom coverage.
To achieve the above object, the present invention provides a process for forming a thin film by means of a sputtering apparatus having a chamber comprising a sputtering space and a film-forming space and a grid plate disposed between the sputtering space and the film-forming space, comprising the steps of:
placing a target and a substrate in the sputtering space and the film-forming space, respectively;
maintaining the pressure in the film-forming space at a pressure lower than the pressure in the sputtering space and a pressure sufficient for sputtered particles to move in the film-forming space with their mean free path which is longer than the distance between the grid plate and the substrate; and
sputtering the target to form a thin film on the substrate.
The present invention also provides a thin-film formation system comprising:
a chamber comprising a sputtering space for placing a target therein and a film-forming space for placing therein a substrate on which a thin film is to be formed;
a grid plate disposed between the sputtering space and the film-forming space; and
a pressure control means for maintaining the pressure in the film-forming space at a pressure lower than the pressure in the sputtering space and a pressure sufficient for sputtered particles to move in the film-forming space with their mean free path which is longer than the distance between the grid plate and the substrate.